Field of the Invention
The present invention relates to a fuel cell system and a cooling control method for the same, and more particularly, to an evaporative cooling type fuel cell system and a cooling control method for the same, capable of cooling a stack in the fuel cell system and humidifying air supplied to a vehicle by adjusting the amount and pressure of air in a cathode based on a stack temperature in the fuel cell system.
Discussion of the Related Art
A fuel cell system is a type of electric power generator, which electrochemically converts chemical energy of fuel directly into electric energy in a fuel cell stack, instead of converting the chemical energy into heat by combustion. Such a fuel cell system mainly includes a fuel cell stack configured to generate electric energy, a hydrogen supply unit configured to supply hydrogen as a fuel to the fuel cell stack, an air (oxygen) supply unit configured to supply oxygen of air as an oxidant required for electrochemical reaction in the fuel cell stack, a thermal management system (TMS) configured to discharge reaction heat from the fuel cell stack to outside fuel cell system, adjust operation temperature of the fuel cell stack, and perform a water management function, and a system controller configured to execute overall operation of the fuel cell system.
The fuel cell system generates electricity by reacting hydrogen as a fuel with oxygen of air. In particular, the fuel cell system generates heat and water as reaction byproducts. The most attractive fuel cell type is an ion exchange membrane fuel cell or a polymer electrolyte membrane fuel cell (PEMFC), which has the highest power density among known fuel cells. The PEMFC is operated at a low temperature to allow a vehicle to be started in a short amount of time. In addition, the PEMFC may have a rapid response time for power conversion.
The fuel cell stack of the PEMFC includes a membrane electrode assembly (MEA), a gas diffusion layer (GDL), gaskets, fasteners, and a bipolar plate. The MEA includes a polymer electrolyte membrane where hydrogen ions are transferred therethrough and electrode/catalyst layers respectively adhered to both sides of the polymer electrolyte membrane. In particular, in the electrode/catalyst layers, electrochemical reaction occurs. The GDL functions to uniformly distribute reactant gases and transfer generated electricity. The gaskets and the fasteners maintain an appropriate airtightness to the reactant gases and coolant and an appropriate fastening pressure. The bipolar plate transfers the reactant gases and coolant. The fuel cell generates current using a fuel cell reaction when hydrogen and oxygen (air) are supplied.
In the fuel cell stack, hydrogen is supplied to an anode, as a positive electrode (or “fuel electrode”), and oxygen (air) is supplied to a cathode, as a negative electrode (or “air electrode” or “oxygen electrode”). The hydrogen supplied to the anode is dissociated into hydrogen ions (protons, H+) and electrons (e−) by a catalyst of the electrode layers adhered to both sides of the polymer electrolyte membrane. Only hydrogen ions selectively migrate to the cathode through the polymer electrolyte membrane as a cation exchange membrane. Simultaneously, the electrons are transferred to the cathode through the GDL and the bipolar plate. In particular, at the cathode, the hydrogen ions supplied through the polymer electrolyte membrane and the electrons transferred through the bipolar plate react with oxygen of air supplied to the cathode through an air supply device, to produce water.
A hydrogen ion flow causes an electron flow through an external conductive wire and then, the electron flow generates a current. In addition, heat is generated in a reaction of generating water. The electrode reactions in the PEMFC may be represented by the following formulas:2H2→4H++4e−  [Reaction in fuel electrode]O2+4H++4e−→2H2O  [Reaction in air electrode]2H2+O2→2H2O+electric energy+heat energy  [Overall reaction]
In the above reaction, the hydrogen ions pass through the polymer electrolyte membrane, and membrane permeability of hydrogen is determined by a function of water amount. Further, as the above reaction proceeds, water is produced, thereby humidifying the reactant gases and the polymer electrolyte membrane.
When a gas is dry, the overall quantity of water produced by the reaction is used to humidify air causing the polymer electrolyte membrane to dry. For appropriate operation of the fuel cell, the polymer electrolyte membrane is maintained moist since membrane permeability of hydrogen is determined by a function of water amount contained in the polymer electrolyte membrane. When the polymer electrolyte membrane is too wet, pores of the GDL are clogged preventing the reactant gases from contacting the catalyst. Accordingly, an appropriate water amount of the polymer electrolyte membrane should be maintained. Further, as an oxidant, ambient air, instead of pure oxygen, is provided to the fuel cell. However, air humidity of ambient air is insufficient to moisten the polymer electrolyte membrane. Accordingly, air should be humidified before being supplied to the fuel cell for appropriate operation of the fuel cell.
Meanwhile, the fuel cell stack has a structure in which a plurality of unit cells are stacked repeatedly. The unit cell is a minimum fuel cell element where hydrogen reacts with oxygen to generate electric energy. The unit cell has a structure in which a bipolar plate, a GDL and an MEA are stacked. The bipolar plate is a core part of the fuel cell, which has various functions such as structurally supporting the MEA and the GDL, collecting and transferring generated current, transferring reactant gases, transferring and removing reaction byproducts, transferring coolant to remove reaction heat, and so on.
As a conventional art related to cooling and humidifying a fuel cell stack discloses that a porous bipolar plate is used to provide system cooling by coolant, reactant humidification and condensate collection. Another related art discloses a fuel cell system and a cooling control method for the same. In particular, water evaporation and water condensation only using air necessary for the fuel cell system are adjusted by adjusting each pressure and each temperature of a cooling channel, a radiator and a cathode. Heat generated from the stack may be radiated and air necessary for reaction may be appropriately humidified.